Christmas light question

I bought a pre-lit tree a few weeks ago and it had been working just fine. The tree consists of the 3 parts. Over the weekend, I noticed the top part only had about half of its lights still lit. The box says that if a bulb is blown or even removed, that the rest of the strand should remain lit. Well this obviously is not the case because there is a single strand on the top portion and a little over half of those lights are not working now.

Fast forward to today - I started pulling each bulb that was not working and inserting a working bulb (that came from another strand on the same tree). After about the 15th try, I hit the bulb that was causing the issue, and the entire strand lit up again. The strange thing is that I noticed the bulb burns much brighter (and a little hotter) than any other bulbs on the same strand. After about 20 minutes, it blew out and I was back to the same problem. Therefore, I do not understand how a light in the middle of the strand can be that much brighter and burn out so fast when the others look fine. I tried replacing the bulb with the spares that came in the box, same effect after 15 to 20 minutes. Any idea as to what has happened to this particular socket? I looked down in the socket and it looks exactly the same as one that works normally. It ticks me off because I have a fully decorated tree and now experiencing a weird problem after it was working fine for several days and it hasn't been touched. Taking the tree down and back to the store would be a huge pain. Thanks for any suggestions.

Right, somehow it's getting more voltage but before the first light blew it worked for a few days, now it's only about 20 minutes. It's very strange to me, something in the socket must have gone bad. I guess my only options are to take it back, or strip the lights off that section and put up some of my own. Only problem is trying to match the brightness of the bottom two sections if I use my own lights. Being that it was a $250 tree, you expect it to work more than a few days. Just the way it goes I guess.

My guess is that there's some kind of shunt device built into the socket that's not working. The shunt is supposed to bypass a burned-out filament and allow the rest of the string of lights to keep working when one bulb burns out. Shunts are not very reliable. You can buy a "zapper" that will sometimes repair bad shunts by applying short pulses of high voltage to the light string. It might be worth trying one of these if it's not feasible to return the tree to the store.

If the shunt goes bad, would it make a replacement bulb shine very bright like it's getting more voltage? I considered the shunt theory and getting one of those zappers, but all of the local stores are sold out. Since none were available, I just pulled the bulbs one at time and found the source of the problem. Can the shunt be replaced easily?

One I worked on had 3 parallel strings of 19 lamps each, in series, each string powered by ~24vac, 57 bulbs total. It was from an IKEA in Germany.
You can stuff tin foil in the socket with the burned out bulb and have all the other lights in the string light up with minimally larger voltage on each bulb. From a distance you didn't notice the bulb was out. I waited for the German authorities to arrest me for this infraction/violation, but they must have been busy elsewhere.

The bulb that was brighter than the rest was not the same voltage rating and/or power rating as the rest.

There is some kind of shunting wire material wrapped around the filament support posts in these bulbs that is supposed to go to a low resistance state if the filament opens. I guess it's triggered by the higher voltage that you get when the filament opens. Or maybe the several turns of special wire is always conductive, so the string draws more power than it would just to light the filaments.
Very clever, when it works.

Wow, The same exact situation happened to me, I ended up replacing about 25-30 bulbs on the top of the tree, and using the "zapper" to try to correct the problem, the zapper did not work, but on that zapper there is a place to check the bulbs, and every bulb I replaced were burnt out...all 25-30 of them. After some time I did replace the burnt bulbs and found the one bulb that made the rest come on. As you said, the top was brighter than the rest of the tree.....I figured it was because the bulbs I used was from a different string of lights from some other X-mas decoration I had.
Then a day later we looked up and the top of the tree was burnt out again...then about 1/2 hour later...back on.
Our tree was only a 6ft one that my wife puts in the kids playroom with their decorations on it...so, we took it down (it was aft. X-mas) and went to Walmart and bought the same one for 50% off...which was $15. for next year. That tree was three years old.

Different count strands are different voltages
Some vary from 2.5-3.5v, maybe more
This also depends if they are energy saver strings or "extra bright" strings. Mixing & matching bulbs will cause them to burn out faster. While other bulbs will remain lit, not replacing the burned out bulbs will cause the entire string to reach "critical mass" & all the remaining bulbs will burn out.
For a pre-lit tree the bulbs "should" all be the same. I have a 7' tree that is prelit with 450 lights. I ma not sure on the exact method, but something like this:
Top section has (1) 75 ct strings
Middle section has (2) 75 ct strings
Bottom section has (3) 75 ct strings

If one socket is burning out bulbs then I assume a Mfg defect & would bring it back. I bought mine at 90% off after Christmas for $30, it was the last one